Centralize ambient colors

[wipheg]

Closes #1506

Summary

• Add AmbientColors as a central place for Discord embed accent colors
• Replace scattered feature-specific color constants with named shared colors
• Reuse matching colors across related features, such as role management and moderation embeds

Testing

• ./gradlew.bat spotlessCheck test

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}